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Notice and InvitationOral Defense of Doctoral Dissertation
Doctor of Philosophy in Computational Science and Informatics
Department of Computational and Data Sciences
College of Science
George Mason University

Theoretical studies of the properties of materials are important as they serve to narrow the focus of what are normally time consuming and costly experimental searches. In modeling these materials, first-principles density functional methods have been proven to quite effective. They have the drawback of being computationally expensive and, to mitigate this, faster approaches have been developed such as the tight-binding model.

We have used the Naval Research Lab (NRL) tight-binding (TB) method to study the electronic and mechanical properties of the noble metals. The tight-binding Hamiltonians are determined from a fit that has a non-orthogonal basis and reproduces the electronic structure and total energy values of first-principles linearized augmented plane wave calculations. In order to perform molecular dynamics simulations, we developed new TB parameters that work well at smaller interatomic distances. We analyze fcc, bcc and sc periodic structures and we demonstrate that the TB parameters are transferable and robust for calculating additional dynamical properties which they had not been fitted to.

To do this, we calculated phonon frequencies and density of states at finite temperature and performed simulations to determine the coefficients of thermal expansion and the atomic mean squared displacement. The energies for vacancy formation were also calculated as were the binding energies for fcc-based, bcc-based and icosahedral clusters of different sizes. The results compared very well with experimental observations and independent first-principles density functional calculations.

Extending from the single element systems, we develop parameter sets for the Cu-Ag and Ag-Au noble metal binary alloys as well. These parameters were fit to the structures 2, 10, 12 − 3,3, with the and representing the different combinations of , and in addition to the fcc , and .

As an output of this extension to the binary systems, the following quantities were reproduced in good agreement with available experimental and theoretical values: elastic constants, densities of electronic states as well as the total energies of additional crystal structures that were not included in the original first-principles database. We also used this TB parametrization for the alloy systems to successfully perform molecular dynamics simulations and determined the energies for vacancy formation, temperature dependence of the coefficient of thermal expansion, the mean squared displacement and phonon spectra. In addition we show that these TB parameters work for determining binding energies and bond lengths of Cu-Ag fcc-like clusters.